Add sd_notify() integration to nginx master process so the systemd unit can
use Type=notify. nginx mainline ships #if (NGX_HAVE_SYSTEMD) guards in nothing
of its own — every distro carries its own patch. This is ours, kept minimal.

Send:
  READY=1     after workers + cache manager are spawned (master enters loop)
  READY=1     again after a successful reconfigure
  RELOADING=1 when reconfigure starts
  STOPPING=1  in ngx_master_process_exit

The build script provides -DNGX_HAVE_SYSTEMD and -lsystemd, so this patch
doesn't touch auto/ configure scripts — only the source.

--- a/src/os/unix/ngx_process_cycle.c
+++ b/src/os/unix/ngx_process_cycle.c
@@ -12,6 +12,10 @@
 #include <ngx_channel.h>


+#if (NGX_HAVE_SYSTEMD)
+#include <systemd/sd-daemon.h>
+#endif
+
 static void ngx_start_worker_processes(ngx_cycle_t *cycle, ngx_int_t n,
     ngx_int_t type);
 static void ngx_start_cache_manager_processes(ngx_cycle_t *cycle,
@@ -132,6 +136,10 @@ ngx_master_process_cycle(ngx_cycle_t *cycle)
     sigio = 0;
     live = 1;

+#if (NGX_HAVE_SYSTEMD)
+    sd_notify(0, "READY=1\nSTATUS=nginx is ready\n");
+#endif
+
     for ( ;; ) {
         if (delay) {
             if (ngx_sigalrm) {
@@ -211,6 +219,10 @@ ngx_master_process_cycle(ngx_cycle_t *cycle)
         if (ngx_reconfigure) {
             ngx_reconfigure = 0;

+#if (NGX_HAVE_SYSTEMD)
+            sd_notify(0, "RELOADING=1\nSTATUS=nginx is reloading\n");
+#endif
+
             if (ngx_new_binary) {
                 ngx_start_worker_processes(cycle, ccf->worker_processes,
                                            NGX_PROCESS_RESPAWN);
@@ -241,6 +253,10 @@ ngx_master_process_cycle(ngx_cycle_t *cycle)
             live = 1;
             ngx_signal_worker_processes(cycle,
                                         ngx_signal_value(NGX_SHUTDOWN_SIGNAL));
+
+#if (NGX_HAVE_SYSTEMD)
+            sd_notify(0, "READY=1\nSTATUS=nginx is ready\n");
+#endif
         }

         if (ngx_restart) {
@@ -655,6 +671,10 @@ static void
 ngx_master_process_exit(ngx_cycle_t *cycle)
 {
     ngx_uint_t  i;
+
+#if (NGX_HAVE_SYSTEMD)
+    sd_notify(0, "STOPPING=1\nSTATUS=nginx is stopping\n");
+#endif

     ngx_delete_pidfile(cycle);